Hydrogen gas is colorless, odorless, and not detectable by human senses. It is lighter than air and hence difficult to detect and is it not detectable by available infrared gas sensing technology. Coupled with the challenge of detection are the safety risks posed by the gas itself.
Hydrogen gas molecules are small and can diffuse through many materials considered airtight. Constant long-term exposure to hydrogen causes a phenomenon known as “hydrogen embrittlement” in many materials including metals and plastics. Embrittlement reduces the ductility and tensile strength of containment vessels to the point of fracture and eventual rupture and makes hydrogen more difficult to contain than other gasses. A form of H2 embrittlement takes place by chemical reaction. At high temperatures, hydrogen reacts with one or more components of metal walls to form hydrides, which weaken the atomic lattice.
Hydrogen gas is colorless, odorless, and not detectable by human senses. It is lighter than air and hence difficult to detect where accumulations cannot occur, and is it not detectable by infrared gas sensing technology. Coupled with the challenge of detection are the safety risks posed by the gas itself. At 1 atm, fire hazards exist for H2—O2 mixtures between the lower flammability limit (LFL) of 4% and upper flammability limit (UFL) of 94% H2 by volume. In air, the lower and upper flammability limit of H2 is 4.1% and 75% H2 by volume, respectively, as shown in FIG. 7 because the O2 composition of air is only 21%.
The lower and upper flammability limit and is also temperature dependent. The minimum ignition energy required to ignite hydrogen gas is between only 0.017 mJ to 1 mJ at 1 atm depending on hydrogen gas concentration in air, and decreases as temperature is increased. In comparison, the typical static electric discharge caused by humans in normal activity and industrial machinery lie the range of 1-100 mJ, thus, all personnel in an enclosed area must be evacuated before the H2 concentration in air reaches the lower flammability limit.
Current commercially available hydrogen gas detection technologies include catalytic, thermal conductivity, electromechanical, resistance based technology, work-function based technology, and optical detectors. Of the commercially available sensor technologies, only resistance and work-function based technologies can be integrated with a compact low-power wireless platform. Acoustic technologies can also be implemented in a passive, wireless configuration, however, none are commercially available.
The operating temperature of solid-state gas sensors is in the range of 50 to 150° C. and is not as hazardous as a catalytic bead sensor. However, the probability of spark discharges increases as humidity decreases and for a given moisture content, humidity is approximately halved for a 10 degree rise in temperature. This suggests that a sensor that operates at elevated temperatures increases the probability of hydrogen combustion via decreasing the minimum ignition energy, the lower flammability limit and increasing the probability of spark discharge.
Another problem with prior art sensor technologies is reversible detection of hydrogen gas at room temperature is difficult because the activation energy required to desorb the hydrogen gas from the sensitive film is a high temperature. Most commercially available hydrogen gas sensors use localized heaters that control the operating temperature, which is typically greater than 300° C. for catalytic bead gas sensors and 50 to 150° C. for solid-state gas sensors. The localized heaters require relatively high constant current, which translates to a limited battery life of the sensor.
The use of surface acoustic wave (SAW) devices as sensors was introduced in the 1970's. The first SAW based hydrogen sensor was demonstrated by D'Amico et al. in 1982. D'Amico utilized SAW single and dual delay line oscillators in order to observe the frequency shift due to mass loading caused by a thick palladium (Pd) film in a range of 1900-7600 Å in the delay path. The fractional change in frequency was found to be proportional to film thickness. The reaction rates ranged from 0.8 to 21 Hz per second depending on gas concentration and flow rate.
Jakubik et al. also implemented a SAW dual delay line oscillator for hydrogen gas sensing, with the distinction of using a bilayer structure in the delay path. The bi-layer structure included a 1200 Å dielectric film consisting of copper phthalocycanine, (CuPc), nickel phthalocycanine, (NiPc), or metal-free phthalocycanine, (H2Pc). The structure was placed between the SAW substrate and a 200 Å Pd film. The dielectric prevented the Pd film from shorting out the acoustoelectric response of the SAW. The mass loading effect of hydrogenated CuPc, NiPc, and H2Pc and 200 Å Pd films are small when compared to the electrical response, thus, the acoustoelectric response is the dominant sensing mechanism.
The devices designed by D'Amico and Jakubik are active and wired and comprise a majority of the SAW-based hydrogen sensing designs found in literature.
A third example is the ball SAW device described in K. Yamanaka, et al., “Ball SAW Device For Hydrogen Gas Sensor,” presented at the IEEE Ultrasonics Symposium, 2003. Like D'Amico, the ball sensor used a 200 Å Pd film in the SAW propagation path. Although, the ball sensor could be configured as a wireless device, the design was relatively complex and expensive to fabricate.
Wireless hydrogen sensors have been demonstrated by Y.-S. Huang, Y.-Y. Chen, and T.-T. Wu, “A passive wireless hydrogen surface acoustic wave sensor based on Pt-coated ZnO nanorods,” Nanotechnology, vol. 21, 2010 used a H2 sensitive resister to modulate a fraction of energy that is reflected by the SAW interdigitated transducer when the resister was exposed to hydrogen gas. Problems associated with Huang H2 sensors include long response time and the devices were not coded, thus when more than one was used, there was no way to distinguish one from another.
Other know hydrogen detectors include U.S. Pat. No. 7,268,662 issued to Hines, et al., on Sep. 11, 2007 which teaches use of a palladium nanocluster thin film deposited on the monolayer an interdigital SAW transducer to cause a modification of a response signal due to a change in conductivity of the palladium film when exposed to hydrogen; and U.S. Pat. No. 7,047,792 issued to Bhethanabotla, et al., on May 23, 2006 teaches nanoparticles or nanowires of palladium and metal free pthalocyanine coated on a lithium niobate substrate of a SAW device delay line.
Articles and papers on the subject include Ralf Kohn, et al, Nanocrystalline mesoporous palladium activated tin oxide thin films as room temperature hydrogen gas sensors, from The Royal Society of Chemistry, 2007 which reports a surfactant-directed assembly approach to form high surface area mesoporous Pd-doped SnO2 films exhibiting an interconnected nanocrystalline structure and high sensitivity for hydrogen gas at room temperature. Another paper by S. Kasthurirengan, et al., Palladium doped tin oxide based hydrogen gas sensors for safety applications AIP Conf. Proc. 1218, 1239 (2010) discloses development of Pd-doped tin-oxide-based hydrogen gas sensors.
The problems associated with the prior art devices described above can be mitigated by the implementation of a wireless, room-temperature hydrogen gas detection system, which continuously monitors multiple nodes and reports temperature and hydrogen gas presence. The ideal solution to the problems includes SAW device coding to determine which SAW device in a multi-tag system detects the hydrogen.